High-efficiency solar cells are very important for use in space power systems for both commercial and military applications. An increased solar cell efficiency is advantageous in reducing satellite mass and launch cost, and also for increased satellite mission lifetime.
The size, mass, and cost of conventional satellite space power systems depends primarily on the optical-to-electrical energy conversion efficiency of the solar cells used. A maximum solar cell energy conversion efficiency is desired to reduce the area of a solar cell array, thereby enabling a greater payload mass and reduced launch vehicle costs. For example, an end-of-life electrical power requirement for a typical geosynchronous communications satellite might be 10 kW. Since the air-mass-zero (AM0) solar energy flux in space is 1.353 kW-m.sup.-2, the 10 kW electrical power requirement would require a solar array panel area of about 50 m.sup.2 when using 20% efficient solar cells. However, by increasing the solar cell efficiency to 40%, the same 10 kW electrical power requirement could be met with a solar array panel of one-half the area and weight.
Since the early 1960's, the paramount goal of the solar cell community has been to improve the energy conversion efficiency of solar cells. Progress towards the development of multi-junction solar cells was first reported in the 1980's. In 1994, a 2-junction InGaP/GaAs solar cell was disclosed with an energy conversion efficiency of 29.5% for incident light from the sun at 45.degree. above the horizon (denoted as AM1.5). (See K. A. Bertness et al., "29.5% Efficient GaInP/GaAs Tandem Solar Cells," in Applied Physics Letters, vol. 65, pp. 989-991, 1994.) In 1996, a 3-junction InGaP/GaAs/Ge solar cell was disclosed with an AM0 (space solar spectrum) energy conversion efficiency of 25.7% (see P. K. Chiang et al., "Experimental Results of GaInP.sub.2 /GaAs/Ge Triple Junction Cell Development for Space Power Systems," in Proceedings of the 25th IEEE Photovoltaic Specialists Conference, pp. 183-186, 1996).
The above 3-junction InGaP/GaAs/Ge solar cell comprises three p-n junctions (i.e. one p-n junction in each semiconductor layer) connected by two tunnel junctions. The resulting structure is a monolithic, series connected, lattice-matched solar cell having three light-absorbing layers with bandgap energies of 1.85 electron volts (eV) for an InGaP layer, 1.42 eV for a GaAs layer, and 0.67 eV for a Ge layer or substrate. The energy conversion efficiency of this 3-junction solar cell is limited by a relatively large 0.75 eV difference in the bandgap energy of the GaAs and Ge materials which results in a significant super-bandgap energy loss to the Ge in the form of heat. Additionally, the energy conversion efficiency of the 3-junction solar cell is limited by a relatively low bandgap energy of the InGaP layer which limits the number of solar photons reaching the underlying GaAs layer and the electrical current produced therein. Since each layer within the solar cell is connected in series, the electrical current limitation of the GaAs layer limits the overall solar cell electrical current that can be produced in response to solar illumination.
Four-junction solar cells have been attempted heretofore by incorporating an InGaAs or a ZnGeAs.sub.2 junction within an InGaP/GaAs/Ge solar cell. In the case of the InGaAs 4-junction solar cell, the overall performance was apparently compromised by a high dislocation density in the mismatched layers of the solar cell structure. The ZnGeAs.sub.2 4-junction solar cell was not successful due to an inability to dope the ZnGeAs.sub.2 n-type to form a p-n junction. Yet another approach to form a high-efficiency 4-junction solar cell has been to mechanically stack a 2-junction monolithic InGaP/GaAs solar cell above another 2-junction InGaAsP/InGaAs solar cell. The resultant mechanically stacked 4-junction solar cell has a theoretical AM0 energy conversion efficiency of about 32-35%, but the approach is not practical due to yield, scale-up, and manufacturing issues.
An advantage of the present invention is that a high-efficiency solar cell is disclosed that provides a substantial reduction in the super-bandgap energy loss compared to the prior art.
Another advantage is that substantially equal electrical currents can be generated within each homojunction of the high-efficiency solar cell of the present invention in response to solar illumination, thereby increasing device efficiency by eliminating restrictions to electrical current flow.
A further advantage of the high-efficiency solar cell of the present invention is that a high energy conversion efficiency of about 40% can be achieved with embodiments of the invention as either a 3-junction solar cell or a 4-junction solar cell.
These and other advantages of the method of the present invention will become evident to those skilled in the art.